Many modern engines and next generation turbine engines require components and parts having intricate and complex geometries, which require new types of materials and manufacturing techniques to produce.
A turbine blade typically includes hollow airfoils that have radial channels extending along the span of a blade having at least one or more inlets for receiving pressurized cooling air during operation in the engine. Among the various cooling passages in the blades, including serpentine channels disposed in the middle of the airfoil between the leading and trailing edges, the airfoil typically includes inlets extending through the blade for receiving pressurized cooling air, which include local features such as short turbulator ribs or pins for increasing the heat transfer between the heated sidewalls of the airfoil and the internal cooling air.
The manufacture of these turbine blades, typically from high strength, superalloy metal materials, involves numerous steps. First, a precision ceramic core is manufactured to conform to the intricate cooling passages desired inside the turbine blade. A precision die or mold is also created which defines the precise 3-D external surface of the turbine blade including its airfoil, platform, and integral dovetail. The ceramic core is assembled inside two die halves which form a space or void therebetween that defines the resulting metal portions of the blade. Wax is injected into the assembled dies to fill the void and surround the ceramic core encapsulated therein. The two die halves are split apart and removed from the molded wax. The molded wax has the precise configuration of the desired blade and is then coated with a ceramic material to form a surrounding ceramic shell. Then, the wax is melted and removed from the shell leaving a corresponding void or space between the ceramic shell and the internal ceramic core. Molten superalloy metal is then poured into the shell to fill the void therein and again encapsulate the ceramic core contained in the shell. The molten metal is cooled and solidifies, and then the external shell and internal core are suitably removed leaving behind the desired metallic turbine blade in which the internal cooling passages are found.
Other jet aircraft engine parts, such as fuel nozzles, have recently been manufactured using AM techniques that involve the buildup of one or more materials to make a net or near net shape (NNS) object, in contrast to subtractive manufacturing methods. AM techniques are capable of fabricating complex components from a wide variety of materials. Generally, a freestanding object can be fabricated from a computer aided design (CAD) model. Applications include direct manufacturing of complex workpieces, patterns for investment casting, metal molds for injection molding and die casting, and molds and cores for sand casting. Fabrication of prototype objects to enhance communication and testing of concepts during the design cycle are other common usages of AM processes.
The increasingly complex internal geometry of aircraft engine parts has led to difficulties in the inspection of produced parts. Moreover, the migration of AM techniques from prototyping operations to full manufacturing production processes has created a need for more advanced techniques for non-destructive testing of the manufactured parts. As the workpieces have increased in size and the internal geometry of the produced workpieces has become more complex, a need has arisen for more powerful radiographic and CT inspection techniques.
The use of contrast agents for medical CT and radiographic inspection is known. CT imaging of manufactured parts relied on zinc iodide contrast agents. See Schilling et al., “X-ray computed microtomography of internal damage in fiber reinforced polymer matrix composites,” Composites Science and Technology 65 (2005) 2071-2078. Zinc iodide is a commonly used contrast agent for CT scanning. CT inspection has been used on certain additively manufactured parts using conventional techniques. See Van Bael et al., “Micro-CT based improvement of geometrical and mechanical controllability of selective laser melted Ti5Al4V porous structures,” Materials Science and Engineering (2011) 7423-7431.
The present inventors have found that traditional CT contrast agents lose their effectiveness in CT inspection as the complexity of the internal geometry and the overall size of the part increases. As advancements in additive manufacturing have led to larger workpieces having more complicated internal geometry, traditional methods of CT inspection lose their effectiveness. There is a need for industrial contrast agents and methods of inspection using these agents, particularly with respect to large-scale additively manufactured parts or precision cast workpiece, that are capable of depositing a contrast agent within the internal geometry of the workpiece, and removing the contrast agent through a process that is non-destructive to the workpiece after inspection.